In the short-stroke honing or superfinishing described in DE-OS 35 33 082, the honing stone is applied to the surface which is to be worked of a rotationally symmetrical workpiece with a specific contact pressure. This is said to result in a certain abrasion of the workpiece with simultaneous smoothing. Further it is said that where the contact pressure applied is only slight the workpiece material removal is also relatively slight and essentially only a smoothing of the workpiece surface takes place. Thereafter, the so-called "release effect" ("Ausklinkeffekt") is said to occur, because the smooth surface of the workpiece is no longer capable of tearing out of the stone structure those cutting grains which have become blunted. However, were the pressure to be considerably increased, the honing stone and the workpiece would be forced into continuous contact because a continuous self-priming of the honing stone would occur, however without the desired smoothing being achieved.
Apart from the last-mentioned disadvantage, a high contact pressure can also be disadvantageous for other reasons. Therefore, the present invention relates also to the microfinishing of workpiece surfaces as carried out with so-called superfinish machines or superfinish equipment. This finishing method lies in the category of "short-stroke honing", and is a process to improve the surface quality or the size precision as well as to produce a defined shape improvement of the workpiece. A machining or chipping with an undefined cutting edge is carried out. This surface finishing belongs in the same category as grinding or lapping. In contrast to grinding, in which rotating tools are used to abrade the workpiece surface, or lapping, which uses loose material, the short-stroke honing process has the following characteristics:
An oscillating honing stone is pressed against a moving workpiece surface with a specific force, whereby, through the grinding effect of the honing stone, the workpiece surface is abraded. The roughness of the workpiece diminishes very quickly during the abrasion process. At the same time, however, the surface of the workpiece influences the surface of the honing stone, whereby the honing stone itself erodes. Long-stroke and short-stroke are the two different methods of honing. Long-stroke honing is often simply referred to as "honing", which makes differentiation between the two all the more difficult. Short-stroke honing is also known as "fine honing", "superfinishing" or "microfinishing". In short-stroke honing, the kinematics of the process and the resulting technological abrasion mechanisms between tool and workpiece differ greatly from the characteristics of long-stroke honing. Therefore, the two processes must be clearly separated. Short-stroke honing differs from long-stroke honing first through a much lower oscillation stroke (0.5 mm to 7 mm compared to strokes of 30 mm and more), and secondly through a higher oscillation frequency.
A further difference between the two processes is that surface refinement is the primary goal of short-stroke honing, whereas with long-stroke honing, the greatest possible surface abrasion per unit time plus surface refinement are desired. Due to the greater abrasion per unit time, a greater roughness necessarily results with long-stroke honing than does with short-stroke honing. In contrast to previous state-of-the-art short-stroke honing, long-stroke honing can be used with positive or form-locking connection or friction-type connection. The positive connection method of long-stroke honing allows better form correction over the friction-type connection method. This was not possible with short-stroke honing until now.
While long-stroke honing is almost exclusively used for interior finishing of cylindrical or similar bores, whereby onto the longitudinal stroke in the direction of the bore axis, a rotational movement of the tool is superimposed, short-stroke honing is mostly used for exterior finishing of rotationally symmetrical workpieces, e.g. rollers and axles, rotationally asymmetrical workpieces such as cams and excenter, for surface finishing of flat surfaces such as tracks and straight edges or rulers, and for interior and exterior finishing of ball bearing inner and outer races.
In no application of short-stroke honing, whether centered or centerless, plunge cutting, passing through or sculpture machining or surface finishing, is it yet possible to achieve a reproducible defined modification in the shape of the workpiece, in addition to surface finishing and improved dimensional accuracy. One reason for this is that in all method variations the tool is held resiliently and frictionally guided against the workpiece and is not constrainedly guided in the infeed direction. Another reason is that, under otherwise constant technological parameters, a maximum workpiece material abrasion results, which is only depending on the tool specification and the starting roughness of the workpiece. This phenomenon, known in the art as the "release effect", always appears when a certain minimum roughness has resulted in the finishing process. At exactly this point, the cutting edge of the honing stone has become so smooth that it no longer cuts, and therefore cannot further abrade the workpiece. If a certain smoothness of the workpiece surface is achieved, the abrasion declines sharply to almost zero.
This "release effect" of the honing stone, observed in various research experiments, is traceable to the blunting of the cutting grains, which, due to the smoothness of the workpiece surface, can no longer be broken out of the bond and replaced by new grains which can cut. Additionally, due to the large contact area between the workpiece and the honing stone, the flushing liquid is not able to remove the workpiece chips and honing stone dust from the pores, and so a passive layer forms between the workpiece and the honing stone.
One disadvantage is the question of after how much abrasion the "release effect" occurs. This depends on numerous influence values, especially the starting roughness of the unfinished workpiece. This can vary greatly, even with mass-produced parts (e.g. differences between dressing cycles of a grinding wheel), so that the degree of material abrasion by the honing stone varies. The results are differences in diameter of the superfinished workpieces, which can be greater than the differences occurring in the previous working stages.
The unstable behaviour of the honing stone is especially disadvantageous during plunge-working of cylindrical parts. Here the honing stone swings out over the edge of the workpiece and primes itself here longer than in the middle. Surface lines with declining ends are the result. This effect is especially distinct in parts with recesses or grooves, such as control pistons, in which the required straight surface lines are hardly achievable.
Therefore, it has already been tried to limit the infeed path of the honing stones with an adjustable stop (DE-OS 35 33 082, see also literature cited in this document and discussion of "release effect" and "self-priming"). As has already been shown in practice, such an adjustable stop can, however, prevent neither the release effect nor self-priming, since there is no relation between the occurrence of these phenomena at the finishing location and the adjustment of the stop.
Apart from the above-mentioned DE-A-35 33 082, reference can also be made to applicant's earlier application DE-A-3 618 274 as well as to the state of the art U.S. Pat. No. 4,558,537, which however, do not lead to entirely satisfactory solutions under exacting or all-round requirements.